The invention concerns a support means for the tread of a tire. Together with the said tire and its mounting rim, the said support means constitutes a rolling assembly for a vehicle, intended to be able to roll after a considerable and unexpected pressure loss from the tire. The said tire is more particularly a tubeless tire of the heavy-duty or civil engineering type.
French Patent Application FR 2 756 221 describes and claims as a means for supporting or holding up a tire tread, a toric membrane made of reinforced rubber, which is inflated to a pressure po higher than the pressure pl of the tire cavity and which, in the inflated condition, has a crown radius RM smaller than the flattened radius RE of the tire used at its recommended pressure, the said membrane, at least at its crown, being reinforced by at least one ply of cords or cables, the said crown of the said membrane comprising in addition at least one hoop-like reinforcement of cords or cables orientated circumferentially and having a rupture strength per cm of ply at least equal to the product of the crown radius RM by the pressure, per cm2, of the area of the said ply, leading to a tension per cm of ply equivalent to the tension due to the maximum centrifugal force to which the tire can be subjected and enabling the rupture of the hoop-like cords or cables at a pressure differential poxe2x88x92pxe2x80x2l, which exists if the tire suffers a pressure loss greater than the initial pressure differential poxe2x88x92pl, in other words when it is rolling normally. The said hoop-like reinforcement may consist of at least one layer, generally located among the crown reinforcement plies, or several strips located in recesses formed on the protective layer radially covering the underlying plies.
The internal pressure po of the said membrane, measured cold, in other words at 20xc2x0 C., is higher than the pressure pl of the internal cavity of the tire by an amount between 0.5xc3x97105 Pa and 5.0xc3x97105 Pa, depending on the dimensions of the tire concerned. Granted that the crown radius RM of the toric membrane is preferably between 0.80 and 0.97 times the flattened radius RE of the tire, mainly for reasons related to the heating of the said tire, too high a pressure difference risks altering a number of properties of the tire itself, for example the endurance of its carcass reinforcement, while also requiring too large a hoop-like reinforcement.
The crown of the said membrane is preferably reinforced by two plies of cords or cables parallel to one another in each ply and crossed from one ply to the next to make an angle with the circumferential direction between 50xc2x0 and 85xc2x0. The cables or cords are advantageously textiles for reasons of lightness, flexibility and good corrosion resistance, and are preferably made of aromatic polyamide. The axial ends of the two plies are preferably located on the side walls of the membrane, such that if the maximum axial width of the tire""s carcass reinforcement is denoted as S, the width of the plies is preferably between S and 1.30 S.
If the tire bursts, the pressure difference poxe2x88x92pl increases, the cables of the hoop-like ply or plies break, the toric support membrane expands inside the tire cavity, and this enables the whole to continue rolling despite the pressure drop in the tire cavity.
Thereafter, such membranes have been the subject of several improvements: the patent FR 2 772 666 concerns the introduction of annular reinforcement elements which enable the wrapping of the side wall reinforcement ply or plies, the said elements being so constituted that they can break at essentially the same pressure difference poxe2x88x92pxe2x80x2l, consequently allowing the membrane to expand within the tire cavity. A second improvement consisted in providing the membrane side wall reinforcement ply or plies with elements formed on the one hand of a core able to rupture at a given pressure difference, and on the other hand of a cord or cable wrapped in a helix around the said core so as to allow a considerable elongation of the side reinforcement elements and thus allow expansion. The use of such cables with force-elongation curves represented essentially by two adjacent straight-line segments with different slopes, has made it possible to adopt several membrane crown reinforcement architectures, the said crown reinforcement being necessary to resist perforation of the membrane itself and being completed by the hoop-like reinforcement of circumferential reinforcing elements designed to keep the crown of the membrane in its initial position, i.e. as for normal rolling with the desired pressures in the membrane and in the tire.
The structures mentioned are not suitable for the conditions set for rolling of such an assembly when the tire cavity pressure becomes zero due to a puncture and the assembly is rolling at a reduced pressure. The resistance of the membrane to a sharp and perforating object undergoing the same rotation movement as the rolling assembly, must be very greatly improved.
To do this, and according to the invention, the toric membrane used as a support means for the tread of a tire P and forming together with the said tire P and its mounting rim J an assembly that can roll when the tire suffers a loss of pressure, the said membrane being expandable, capable of filling the internal cavity of the tire, consisting of a cap reinforced by a hoop-like armature and connected via two side walls reinforced by a carcass reinforcement to two beads, each bead comprising at least one inextensible and strong reinforcement ring around which the carcass reinforcement is anchored by forming a turn-up, is characterized in that radially above in internal layer of rubber impermeable to the inflation gases there is a crown reinforcement of at least two plies of textile reinforcement elements crossed from one ply to the next and making with the circumferential direction angles between 40xc2x0 and 75xc2x0, the said crown reinforcement being detached from the said internal layer over a meridian distance equal to its width and being radially overlaid by the edges of two carcass half-plies which extend in the side walls while being detached from the said edges, each carcass half-ply being turned up in each bead around a reinforcement ring and being formed of textile reinforcement elements orientated relative to the circumferential direction at an angle between 40xc2x0 and 75xc2x0, the said edges on either side of the equatorial plane being connected radially on the outside by at least one connecting ply of textile elements, the combination of the said edges and the connecting ply being radially overlaid by a hoop-like reinforcement consisting of at least one ply of circumferential textile elements, the reinforcement elements of the hoop-like reinforcement and the elements of the connecting ply having strengths which allow them to break at a pressure differential poxe2x88x92pxe2x80x2l that exists in the event of pressure loss in the internal tire cavity, this being larger than the initial pressure differential poxe2x88x92pl during normal rolling.
The hoop-like reinforcement is advantageously partially detached from the combination formed by the carcass half-plies and the ply connecting the edges of the said half-plies, and this combines two advantages, on the one hand that the reinforcement elements of the hoop-like armature are not dispersed after breaking, and on the other that the crown of the support membrane is as flexible as possible.
A reinforcement (armature), ply or layer is said to be detached from a reinforcement (armature), ply or layer when the reinforcement, ply or layer does not adhere at all in the vulcanized condition to the reinforcement, ply or layer radially adjacent to it, the said absolute lack of adhesion being obtained by using an anti-adhesion product before vulcanization, for example a stearate-based solution and metallic powder.